Humans have kept animals around for just about as long as we’ve been human. Dogs helped us hunt. Cats guarded the granaries.

But the notion of having animals strictly as companions, as opposed to four-legged workers, wasn’t too common until an economic middle class — that stratum between the 1 percent and the serfs — came into its own in the 19th century. That meant a lot of people had the resources to own and take care of animals that weren’t, strictly speaking, useful.

And with the middle class came the idea of pets: Animals with names and individual personalities. Animals we care about for other than utilitarian reasons.